By aspect — in detail
Bass
Strong consensus · 11 srcThe single strongest point of agreement in the whole corpus, and the reason most reviewers recommend it: deep, weighty, textured, and kept clear of the mids. The only recurring reservation is character rather than quality — a minority hear the attack as soft and meaty rather than taut, which is also part of why the rest like it.
“Bass extension is excellent here and note also how it hedges the line between being a warmer tuning and a more sub-bass focused one.”
TechPowerUp (VSG)
“for a sense of authenticity to bass notes, the U4s certainly delivers with a meaty thickness and plenty of decay”
Headphones.com (Theo Lee / Precogvision)
“The texture is excellent, offering bass that doesn’t just hit hard, it also provides a clear tight presentation.”
Bloom Audio
“it sports a more pillowy characteristic to bass attack”
Headphones.com (Theo Lee / Precogvision)
Measured
Public IEC-711 exports put the bass shelf at +9.4 dB at 30 Hz over the ~450 Hz minimum with the stock m15 module, and +9.5 dB with m20 — the two bassiest modules differ far more in isolation than in bass. m12 drops it to +6.8 dB and mX to +3.0 dB, close to flat. TechPowerUp's independent rig read the m15 shelf at "~9.5 dB of bass boost with the m15 Apex modules here relative to the lowest point at ~500 Hz".
Sources agree on the shape and split on the verdict. The ear gain peaks early and modestly, so vocals — sopranos especially — sit further back than the bass and treble. One camp reads that as a deliberate, non-shouty midrange; the larger camp reads it as vocals stripped of presence. Swapping m15 for m12 lifts the mids by taking bass away, which is why the split partly tracks which module each writer used.
Measured
Three independent IEC-711 rigs agree that the pinna compensation peaks near 2 kHz rather than the usual 3 kHz, with a relative trough just above it — the export used here reads +5.7 dB at ~2.05 kHz falling to +2.5 dB at ~3.2 kHz, and KillerLab's own graph shows "an early peak at around 2.15 kHz, followed by a 3.3kHz dip". On the same rig the U12t carries about 1.5 dB more ear gain, which is the measured reason its vocals step forward by comparison.
⚠ vs. listeners — The graphs settle what is physically there and cannot settle the argument: a shelved-back upper midrange is exactly what a listener who hates shout wants and exactly what a listener who wants vocals up front does not.
Where it splits
recessed — vocals sit back and lose body and presence67%
“Female vocals come off slightly blunted with higher-pitched notes sounding hollow.”
TechPowerUp (VSG)
The most polarizing part of the in-ear, and the reviewers say so themselves. The tia driver buys real extension and air, and most writers found it detailed without turning harsh. A consistent minority hear the same response as uneven — thin, glassy cymbals and fatigue at volume. Both camps describe the same measured shape; insertion depth and treble sensitivity decide which one you land in.
Measured
The exports show a broadly lifted mid-treble rather than one spike — about +5.3 dB at 4.55 kHz and +5.5 dB near 7 kHz relative to 1 kHz — with the level falling away above roughly 10 kHz. Coupler readings past 8 kHz are not reliable, which is precisely the region both camps argue about.
⚠ vs. listeners — KillerLab, who owns the set and graphed it, reports the split directly — "People around me either love or hate this tuning" — and names a lever nobody else does: "the deeper the insertion, the less treble detail, but at the same time, the smoother it gets, and thus, less fatiguing." Same in-ear, same graph, different depth in the ear canal.
Where it splits
airy and well extended without turning harsh76%
“I suppose the strong point for me was the treble response here which comes off very well extended without being overly harsh, sibilant, shouty, or fatiguing in any sense.”
TechPowerUp (VSG)
uneven and thin up top — sharp rather than silky, and fatiguing at volume24%
“Some higher notes, especially hard and fast hitting cymbals, can come across as thin.”
Headfonia (Felix)
Tonality
Strong consensus · 7 srcConsistently described as bass-boosted and mildly U- or V-shaped, sitting on the bright side of neutral rather than the warm side, and coloured on purpose. Writers reach for different labels — "neutral with a bass boost" from the bench, "a mild V iem" from a measurer's ear, and one owner noting it strays "far away from the descriptor: neutral" — but they are all describing the same curve.
“The overall sound signature of the U4s can be seen as neutral with a bass boost”
TechPowerUp (VSG)
“The U4s is more U-shaped than L-shaped”
Headphones.com (Theo Lee / Precogvision)
Soundstage
Contested · 9 srcGenuinely split, and the split has a physical cause. Roughly two-thirds of the weight calls the presentation open and unusually tall; the rest find it a small room with limited depth. Two sources independently describe the mechanism: a tighter Apex seal adds bass and sharpens imaging but shrinks the stage, while the open modules widen it and blur the focus. Foam tips close it down further.
Measured
The Apex module is a stage control as much as a bass control: Bloom Audio notes that "m20 brings in the soundstage a bit, but also adds a harder hitting bass", while mX opens it out. The reviewers who heard a small stage were on m15 or m20; those who heard a big one were on m12 or mX.
Where it splits
open and notably tall — an airier presentation than expected72%
“The tia high gives the U4s a relatively tall-sounding monitor. It’s definitely an airier presentation than what I was expecting”
Headfonics (Marcus Downes)
modest — a smaller room, with depth and width the weak points28%
“The U4s creates a smaller room where depth and width could be better.”
Headfonia (Felix)
Solidly good, and short of class-leading by common consent. Separation and centre placement draw praise; the recurring caveat is that 64 Audio's own all-BA sets, and some rivals at the price, place instruments more precisely.
“Imaging is excellent with the m15 and m12 modules”
TechPowerUp (VSG)
“Imaging on the U4s is also decent even if it stumbles somewhat in sharpness of instrument positioning and layering.”
Headphones.com (Theo Lee / Precogvision)
“the real standout of the U4s is its imaging”
Joshua Patton
Resolves well, led from the top: the tia driver puts a lot of edge definition on percussion and cymbals. The dissent is about the middle — the same reserve that pulls vocals back also makes midrange detail harder to pick out, and the measurement-minded reviewer is blunt that it is not the most resolving set at the money.
“Resolution is another strong suit of the U4s”
Head-Fi showcase (KillerLab)
“Not the most resolving set for the money”
TechPowerUp (VSG)
Dynamics
Strong consensus · 6 srcUncontested. Every source that discusses it describes notes landing with force and contrast — the dynamic driver's physicality is what separates it from 64 Audio's all-BA sets. The one qualification is that the impact is weighty rather than sharp-edged.
“Macrodynamics on the U4s are exemplary, notes sounds explosive and alive.”
Head-Fi showcase (KillerLab)
“Dynamics on the U4s are praiseworthy.”
Headphones.com (Theo Lee / Precogvision)
Most reviewers wear it for hours without thinking about it — it is light, about 6 g a side, and the Apex vent removes the pressure build-up that makes sealed in-ears tiring. But the shell is a universalised custom mould with a long, narrow nozzle, and listeners whose ears differ from that mould report contact points and fiddly seals. There is a second, less obvious trap: the dynamic driver vents through the top of the shell, and an ear that covers it changes the bass.
Where it splits
fit depends on your ear shape — contact points and a finicky seal37%
“But 64Audio’s UIEM design touches my antihelix, causing slight discomfort over time.”
Head-Fi showcase (KillerLab)
Isolation
Strong consensus · 7 srcAgreed, and agreed to be a trade-off rather than a flaw. Apex is a vent, so the U4s cannot reach the attenuation of a sealed in-ear; the modules are named for what they give up, from about −20 dB (m20) to about −10 dB (mX). Reviewers call it adequate rather than good, and recommend foam tips with m20 if noise is the priority.
“It’s a vent, so it simply cannot match the standard -26dB isolation created by a sealed IEM.”
Headphones.com (Theo Lee / Precogvision)
“neither the silicone nor the foam tips did that well for noise attenuation on their own”
Headfonics (Marcus Downes)
The materials are not in question — machined aluminium, and one reviewer dropped his on a stone floor without a mark. What splits opinion is the finishing and what comes in the box at this price: a dissenting owner found machining marks, a faceplate seam and a rough nozzle junction on his unit and on two demo pairs, and both critical voices single out the accessories — a single-ended-only cable, thin stock tips, a fragile shirt clip, and replacement Apex modules at about $100 a pair.
Measured
The one build claim that is measured rather than felt goes the U4s's way: channel matching is excellent. TechPowerUp found the two channels "practically identical across the entire useful 20 Hz to 20 kHz range" on a randomly drawn retail sample, and KillerLab's separate rig put left-right difference under 1 dB across most of the audible band.
Where it splits
flawless — machined aluminium, and it takes a beating74%
“When it comes to build quality, I have to admit, that 64 Audio is doing a perfect job.”
Headfonia (Felix)
finishing and accessories don't live up to the price26%
“the build quality on the earpiece is a bit middling in my opinion”
Head-Fi showcase (KillerLab)
The consensus is that it undercuts its own family. Reviewers repeatedly note it does what the $1,700 Nio did for far less, and several say plainly that it made the $2,000 U12t harder to justify. The dissent is that a kilobuck is still a kilobuck: the community aggregate's most common complaint is price for performance, and the list price has since moved up.
“I truly think the U4s is a new benchmark at the $1000 price range and possibly higher too”
TechPowerUp (VSG)
“the U4s effectively undercuts the Nio by a third of the price with comparable (again, if not better) sound quality”
Headphones.com (Theo Lee / Precogvision)
“And after over a month listening to it, I gotta say it's one of their best.”
Super* Review
